This invention relates generally to the investment casting of metals and alloys that contain metal silicides. More specifically, it relates to the investment casting of niobium-silicides in shell molds.
Many different types of metals and metal alloys are especially useful for high temperature equipment, e.g., engines and other machinery. As one example, superalloys have been the materials of choice for turbine engine components, such as turbine buckets, nozzles, blades, and rotors. The superalloys are often based on nickel, although some are based on cobalt, or combinations of nickel and cobalt. These materials provide the chemical and physical properties required for turbine operating conditions, i.e., high temperature, high stress, and high pressure. As an illustration, an airfoil for a modern jet engine can reach temperatures as high as about 1100° C., which is about 80-85% of the melting temperature of most nickel-based superalloys.
While nickel-based superalloys continue to be tremendously popular, research efforts in recent years have also focused on alternative materials for high temperature components, such as the turbine engines. Refractory metal intermetallic composite (RMIC) materials are a prime illustration. Examples include various niobium-silicon alloys (sometimes referred to as “niobium-silicides”). (The RMIC materials may also include a variety of other elements, such as titanium, hafnium, aluminum, and chromium). Such materials generally have much greater temperature capabilities than the current class of superalloys. The melting point for a metal charge based on the RMIC materials will of course depend on the individual constituents of the RMIC, but is usually in the range of about 1500° C. to about 2100° C.
RMIC materials, as well as the superalloys, are cast into useful articles by various techniques. One of the most popular techniques is investment casting, sometimes referred to as the “lost wax process”. Typically, the process involves dipping a wax model into a slurry comprising a binder and a refractory material, so as to coat the model with a layer of slurry. The binder is often a silica-based material. Colloidal silica is very popular for this purpose, and is widely used for investment-casting molds. Commercially available colloidal silica grades of this type often have a silica content of approximately 10%-50%.
Typically, a stucco coating of dry refractory material is then applied to the surface of the slurry layer. The resulting stucco-containing slurry layer is allowed to dry. Additional stucco-slurry layers are applied as appropriate, to create a shell mold around the wax model having a suitable thickness. After thorough drying, the wax model is eliminated from the shell mold, and the mold is fired.
The ceramic shell molds used during investment-casting must exhibit a number of important attributes. For example, the strength and integrity of the mold are very important factors in ensuring that the metal part formed in the mold has the proper dimensions. These attributes are especially critical for manufacturing high performance components, such as superalloy parts used in the aerospace industry.
The shell molds described above are very suitable for casting in many situations. However, considerable drawbacks are sometimes present. For example, free silica in the shell mold tends to limit the casting temperature and the materials which can be successfully cast. Other problems are present when the shell mold is used to cast chemically-reactive materials like the niobium-silicides. As an illustration, silica in the wall of the shell mold can react with the niobium-silicide material, resulting in serious surface defects in the cast article. Precision casting is limited because of the defective surfaces. In some cases, over-size parts must be cast and then machined-to-size in order to remove the surface defects.
Facecoats are sometimes used to form a protective barrier between the molten casting metal and the surface of the shell mold. For example, U.S. Pat. No. 6,676,381 (Subramanian et al) describes a facecoat based on yttria or at least one rare earth metal and other inorganic components, such as oxides, silicides, silicates, and sulfides. The facecoat compositions are most often in the form of a slurry which includes a binder material, along with a refractory material like the yttria component. When a molten, reactive casting metal is delivered into the shell mold, the facecoat prevents the undesirable reaction between the casting metal and the walls of the mold, i.e., the walls underneath the facecoat. Facecoats can sometimes be used, for the same purpose, to protect the portion of a core (within the shell mold) which would normally come into contact with the casting metal.
Yttria is a very desirable component for the facecoat slurries, because of its refractory-nature, and chemical inertness. In fact, yttria-based slurries have been evaluated to some degree in the past, as described in U.S. Pat. No. 4,947,927 (Horton). Unfortunately, there are serious problems associated with yttria slurries of this type, in regard to both the facecoat and the remainder of the shell mold structure. The slurries are chemically and thermally unstable, making them difficult to store and use. They can also be expensive to prepare. Furthermore, as described in the Horton patent, the use of yttria-based slurries can lead to a facecoat surface which has considerable imperfections, such as pores and pits.
It should thus be apparent that improved shell molds which can accommodate high-temperature materials like the niobium-silicides would be welcome in the art. The shell molds should have refractory surfaces (e.g., in the form of facecoats) which are relatively inert to the high temperature materials being cast. Moreover, the shell molds should be capable of being prepared economically from slurries, using an investment casting process. The shell molds should also be capable of accommodating pre-fabricated cores which are fully compatible with materials like the niobium-silicides. Furthermore, it would also be very advantageous if the physical properties of the walls of the shell mold could be adjusted throughout their thickness, e.g., in terms of wall strength and thermal expansion characteristics.